In my co-pending U.S. patent application Ser. No. 695,411, now U.S. Pat. No. 4,094,487 there is disclosed a device for securing to a post a body provided with a channel member having a restricted mouth, the device comprising an elongate plate for insertion lengthwise into the channel to extend across its mouth and be retained in the channel by the inturned lips thereof, a pair of attachment members connected or arranged for connection to the elongate member through the mouth of the channel and connected or arranged for connection to a strap to be passed around the post. This device, while being very strong, extremely difficult to force out of the channel, and easy to use on site, has certain disadvantages. The spacing of the attachment members is fixed, so it is necessary to have different sizes of device for different size posts unless an intermediate saddle is used. The full length insert plate represents a substantial part of the materials cost, and the device requires at least two welds which add to the labour and time costs in manufacture. Also, it is sometimes inconvenient on site to have a unitary construction of that type for insertion into the channel as a single unit. The present invention seeks to ameliorate some or all (depending on the particular embodiments) of the above disadvantages.